Producing short optical pulses from semiconductor lasers with the intention of further amplifying the pulses in fiber amplifiers and using the pulses in applications such as materials processing puts a strong requirement on a seed source with low levels of CW amplified spontaneous emission (ASE), which if present would be amplified in the amplifier chain and produce amplifier emission between optical pulses. This emission can cause deleterious effects to sensitive materials in some applications and generally causes confusion about the true peak power of a laser. Measurements of pulse power in the presence of CW ASE may be mischaracterized as being associated with pulse power. Undesirable ASE also steals power from the pulses and thus causes a need for higher pump power for achievement of similar pulse energies in a given amplifier.
The cause of this ASE in conventional gain switched diodes (DFB, DBR, VBG or FBG externally stabilized and others) used to produce short optical pulses lies in the drive electronics mechanism. Conventionally such diodes are driven with a short electrical pulse and application of a DC bias current of a few to a few tens of mA. Under many driving conditions, this bias current results in ASE from the semiconductor material in a CW fashion, as the bias excites the semiconductor material such that it emits light continuously, but does not drive the device over laser threshold. This light is broadband, and much of it can be filtered by conventional spectral filters in an amplifier chain. However ASE in the passband of such filters (which for practical reasons are ˜1-2 nm wide) cannot be removed, and results in a CW component in the laser output. This problem becomes increasingly worse as laser repetition rate is lowered, as the DC bias produces a constant amount of ASE regardless of pulse repetition rate. As pulse repetition rate is decreased, fewer optical pulses are emitted in a given time period and the ASE becomes an increasingly greater percentage of the output power. In extreme cases, ASE can be >90% of the output power (before spectral filtration) and >10% of the output power (after spectral filtration) when laser repetition rates are less than a few hundred kHz. These rates are desirable for materials processing and LIDAR transmitter applications, among others, so systems having reduced ASE are needed.